In adult animals including man, variations in body weight tend to be small over long periods of time. However, there is considerable variability between individuals in their relative resistance to gaining weight. Some organisms gain weight more easily than others. In most cases, these differences in body weight gain cannot be attributed to caloric intake. Neumann created the term luxuskonsumption to describe the wasting of excess calories as heat during overfeeding. Aerobic capacity refers to the capacity of lean body tissue, particularly skeletal muscle, to utilize oxygen for energy metabolism. We have hypothesized that luxuskonsumption evolved secondarily to areobic capacity in man and animal in response to increased demands for endurance performance. It is known that aerobic training such as running can dramatically increase aerobic capacity. We have experimental evidence that shows that not only is luxuskonsumption highly related to aerobic capacity in a variety of people, but that with aerobic training it can be enhanced. Two studies with rats have been proposed that would further test our hypothesis and allow us to directly determine the role muscle metabolism may play in energy balance. Specifically, the two studies will involve training animals on a treadmill at a fixed percentage of their aerobic capcity. The effects of this training on luxuskonsumption in the whole animal will be determined using indirect calorimetry. The metabolism of muscle and brown adipose tissue will be assessed using tissue respirometry. The effects of detraining on thermogenesis will be studied. This research will help to clarify the importance of exercise in etermining body consumption, and may also have implications for how exercise may be used in the prevention and treatment of obesity.